1. Field of the Invention
This invention relates to a non-polar solid capacitor, and more particularly, it relates to a non-polar film capacitor having a layered structure in which an insulating polymer thin film as a dielectric is formed on the porous surface of a conductor and an electrically conductive layer as a counter electrode is formed on the insulating polymer thin film.
2. Description of the Prior Art
Conventional film capacitors are produced in such a manner that a polyester or polypropylene film as a dielectric is interposed between the metal electrodes such as aluminum foils and wound together, or alternatively, a metal thin film is deposited on one or both surfaces of the polyester or polypropylene film, followed by winding or layering of the composite film. In general, to obtain a film capacitor with high capacitance, a plastic film having a metal thin film deposited on one or both surfaces thereof is used in a wound or laminated form. However, the production of extremely thin plastic films has a limitation in an industrial scale. For this reason, it is difficult to obtain film capacitors with high capacitance, and film capacitors used for actual applications are therefore limited to capacitance of the order of several microfarads.
In the case of electrolytic capacitors using valve metals which can be readily anodized, such as aluminum or tantalum, large surface areas can be achieved by making the surface of these metals porous and then extremely thin dielectric oxide layers can be formed on the porous surface of the metals, thereby obtaining capacitors with high capacitance. However, dielectric oxide layers are fragile and therefore weak with respect to mechanical stresses applied in winding. Because numerous defects are present in the oxide films, the leakage current flowing through the capacitors becomes large. Furthermore, dielectric oxide layers formed from valve metals have a rectifying action and it is therefore difficult to produce non-polar capacitors by this method.
Because the use of a dielectric having a large surface area is preferred for the purpose of obtaining capacitors with high capacitance, the method in which the surface of a conductor as an electrode is made porous and a dielectric layer is formed on this surface may be considered. For example, a non-polar capacitor with high capacitance can be produced by forming an insulating polymer thin film as a dielectric on the porous surface of a conductor which has been treated to have an increased surface area, such as an etched aluminum foil. However, it is difficult to form an insulating polymer thin film which has a configuration corresponding to the surface irregularities of the conductor without filling the pores of the conductor having an increased surface area.
To give an example of dielectric materials, polyimide resins have excellent thermal resistance and excellent electrical characteristics and should therefore be regarded as a promising dielectric material for capacitors. However, it is difficult to obtain free-standing thin films from polyimide resins and solid capacitors using polyimide films have not yet been put to practical use. For the purpose of applying polyimide films as a dielectric, if a conductor which has been treated to have an increased surface area is first immersed in a solution containing polyamic acid so that the micropores of the conductor is filled with the solution, after which the polyamic acid is dehydrated to form a polyimide resin, then the micropores are filled with the polyimide resin and the surface of the conductor is made substantially flat. Because a polyimide film cannot have a configuration corresponding to the surface morphology of the conductor, the increased surface area of the conductor cannot be effectively utilized, and consequently the desired high capacitance cannot be obtained.
Even if an insulating polymer thin film could be formed on the porous surface of a conductor which has been treated to have an increased surface area, in such a manner as to have a configuration corresponding to the surface morphology of the conductor without filling the pores of the conductor, then a counter electrode layer in close contact with the insulating polymer thin film cannot be formed from a metal foil or deposited metal layer due to the similar porous state of the insulating polymer thin film, and consequently the desired high capacitance cannot also be obtained.